Mechanical Behavior of L1 2 Single Crystal Al 66 Ti 25 Mn 9
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MECHANICAL BEHAVIOR OF L1 2 SINGLE CRYSTAL A16 6TI 2 5Mn9 S. A. BROWN*, D. P. POPE** AND K. S. KUMAR*
*Martin Marietta Laboratories, Baltimore, MD 21227
"**Dept. of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104 ABSTRACT
Single crystal AITi 6 25Mn9 has been produced and tested in compression as a function of temperature and orientation. Yield strengths for orientations near [001] and [Till continuously decrease with increasing temperature in a manner similar to other single crystal Li2 trialuminides, and also to polycrystals of these materials. Slip was determined to occur on the {111} octahedral planes using two-surface analysis. Critical resolved shear stress (CRSS) variation with temperature, calculated on (111) planes, overlap closely for both orientations. Dislocation analysis confirmed the Burgers vectors to be of the type a at both 298K and 1073K. A limited number of uniaxial tension tests were conducted near [001] at 1073K; the HIPed specimens contained a small amount of residual porosity and second phases which resulted in elastic failure even at this high temperature. INTRODUCTION
The mechanical response of ternary L12 trialuminides derived by macroalloying binary AI3Ti (DO22 structure) with any of several transition elements has generated interest in their deformation mechanisms and fracture behavior [1-9]. Production of completely single-phase microstructures, however, has been difficult in these systems; frequently second phases such as AI2Ti are present [3, 4]. Polycrystals of these compounds have been tested in compression, bending and intension as a function of temperature; low temperature tensile ductility has been confirmed only in the Cr- and Mn-alloyed variants thus far [5-8]. In addition, isothermally forged Al66Ti25Mn9 tested in uniaxial tension revealed a ductility minimum at -773K [5]. Reasons for the lack of ambient ductility and the occurrence of the intermediate temperature ductility minimum are not yet clear. The potential role of grain boundaries in impeding slip transfer from one grain to another at low temperatures and the possibility of these boundaries being embrittled at the intermediate temperatures, leading to a ductility minimum, prompted the present study. In this study, single crystals of A166Ti25Mn9 were characterized interms of their microstructure and mechanical properties with the intentions of identifying the role of grain boundaries, or lack thereof, on ambient temperature ductility as well as on the intermediate temperature ductility minimum. To date, compressive yield strength variation with temperature and orientation has been determined and operative slip systems identified. Persistence of a small amount of porosity in the HIPed single crystal has limited tensile testing to elevated temperatures (1073K) and even then, failure occurred elastically. EXPERIMENTAL PROCEDURE
Cylinders (19 mm diameter) were machined from a 3-kg casting of nominal composition AI8 6Ti25Mn9 and used for single crystal growth via a modified Bridgeman
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